In general, in image-forming apparatuses utilizing an electrophotographic method (including an electrostatic recording method), such as a copy machine, a facsimile, and a laser beam printer, an image-forming process includes an exposure step of performing an image exposure on a photosensitive member to form an electrostatic latent image; a development step of applying a toner (colored resin particles) to the electrostatic latent image to form a toner image; a transfer step of transferring the toner image onto a material to be transferred such as a recording sheet or an overhead projector (OHP) sheet; a fixing step of fixing the unfixed toner image on the material to be transferred by means of, for example, heating and pressing; a cleaning step of removing untransferred toner remaining on the photosensitive member; and the like.
A general fixing method is a method of fusion-bonding the unfixed toner image formed on a material to be transferred onto the material to be transferred by heating and pressing. As a method of heating and pressing, the following method has been employed. A fixing roller and a pressure roller are disposed so as to face each other, a material to be transferred having an unfixed toner image thereon is allowed to pass between the rollers, thereby pressing the material between the rollers and heating with the fixing roller including a heating source therein. The fixing roller has a structure in which a cylindrical metal core is used as a base member, the outer surface of the metal core is coated with a rubber or a resin having a good mold-releasing property, and a heating source such as an electric heater is disposed in the hollow part of the metal core. However, a fixing roller having such a structure requires a relatively long waiting time to increase the surface temperature of the fixing roller to a fixing temperature with the heating means disposed in the hollow part thereof.
On the other hand, as shown in FIG. 1, a method has been developed in which an unfixed toner image 5 formed on a material 4 to be transferred is fixed by heating using heating means (heater) 2 with a fixing belt 1 composed of a thin endless belt therebetween. More specifically, the heater 2 and a pressure roller 3 are disposed so as to face each other with the thin fixing belt 1 therebetween, and the material 4 to be transferred having the unfixed toner image 5 thereon is allowed to pass therethrough while heating with the heater 2. Thereby, the unfixed toner image 5 on the material 4 to be transferred is fusion-bonded on the material 4 to be transferred. The heater 2 is fixed, and the fixing belt 1 and the pressure roller 3 rotate in directions opposite to each other. A pressure roller having a structure in which a rubber layer and a fluorocarbon resin layer are arranged on a metal core in that order is generally used as the pressure roller 3. A method using a pressure belt instead of the pressure roller 3 has also been developed.
In the fixing method using a fixing belt, the unfixed toner image 5 on the material 4 to be transferred is heated using the heater 2 with the thin fixing belt 1 therebetween. Accordingly, the temperature of a fixing unit reaches a predetermined fixing temperature within a short time, and the waiting time after the starting of an electric power supply can be extremely decreased. In this fixing method, only a portion necessary for fixing is heated, and thus the electric power consumption is also small. The fixing belt used in this fixing method requires good heat resistance, mechanical strength, mold-releasing property, and the like.
In consideration of the above required performances, a fixing belt in which a polyimide tube functioning as an endless belt is used as a base member and a fluorocarbon resin layer is provided on the outer surface of the base member has been generally used. However, such a fixing belt including a polyimide tube as a base member is composed of a polyimide resin having a low thermal conductivity and a fluorocarbon resin. Accordingly, an improvement in a thermally conductive property has been desired in order to increase the fixing speed, to decrease the fixing temperature, and to further improve the fixing property of a full-color toner. In order to improve the thermally conductive property of the fixing belt, when the thickness of the polyimide tube is decreased, the mechanical strength thereof decreases. Consequently, for example, wrinkles, flattening, and cracking of the fixing belt are readily formed by rotations during fixing.
Japanese Unexamined Patent Application Publication No. 62-3980 (Patent Document 1) proposes an endless belt including an endless belt made of a polyimide resin, and an inner layer made of a polyimide resin composition containing a thermally conductive filler, the inner layer being disposed on an inner peripheral surface of the endless belt. However, in this endless belt, since the thermally conductive property of the endless belt made of the polyimide resin is insufficient, the improvement in the thermal conductivity in the thickness direction is not sufficient. In addition, since the endless belt requires a step of forming the inner layer, the cost increases.
Japanese Unexamined Patent Application Publication No. 3-25478 (Patent Document 2) discloses an endless film made of a resin composition in which an inorganic filler such as carbon black, silicon carbide, or silica is mixed with a polyimide resin. According to a fixing belt including the endless film as a base member, the fixing temperature can be decreased to a certain degree, but elongation of the fixing belt drastically decreases when the mixing ratio of the inorganic filler increases. Accordingly, Examples in Patent Document 2 disclose only endless-belt-shaped polyimide films prepared by incorporating carbon black, silicon carbide, or silica in a polyimide resin at a low mixing ratio in the range of 1 to 5 weight percent. However, when the mixing ratio of the inorganic filler to the polyimide resin is too low, it is difficult to sufficiently increase the thermal conductivity of the resulting polyimide tube.
Japanese Unexamined Patent Application Publication No. 8-80580 (Patent Document 3) discloses a fixing belt including, as a base member, a polyimide tube made of a resin composition in which an inorganic filler having a good thermally conductive property, such as boron nitride, is incorporated in a polyimide resin. Although Patent Document 3 discloses the mixing ratio of boron nitride over a wide range, in reality, it is necessary to incorporate boron nitride at a high ratio in order to obtain a fixing belt having a satisfactory thermally conductive property. When the content of the boron nitride in the polyimide resin is too low, the thermal conductivity of the fixing belt cannot be satisfactorily increased, and thus it is difficult to achieve required performances such as an increase in the fixing speed. In addition, when the content of the boron nitride in the polyimide resin is too low, the modulus of elasticity does not also sufficiently increase, and thus twist deformation of the fixing belt and flattening at an end of the fixing belt readily occur.
However, although an increase in the boron nitride content in the polyimide resin improves the thermal conductivity of the fixing belt, a mechanical strength such as the tensile strength significantly decreases. During fixing, the shape of a fixing belt is repeatedly changed in a portion that is in contact with a heater, and in addition, the fixing belt is rotated while both edges of the fixing belt are in contact with a holding jig. Therefore, when the tensile strength of the fixing belt is too low, breaking of the fixing belt and cracking at the edges of the belt readily occur. These problems become serious when both the tensile strength and the modulus of elasticity in tension of the fixing belt decrease.
Japanese Unexamined Patent Application Publication No. 2004-123867 (Patent Document 4) discloses a polyimide tubular product (polyimide tube) containing 0.1 to 100 parts by weight of carbon nanotube relative to 100 parts by weight of a polyimide resin and having a thermal conductivity of 0.30 W/m·K or more. Although Patent Document 4 discloses a mixing ratio of carbon nanotube over a very wide range, Examples in the patent document describe that a high mixing ratio of carbon nanotube in the range of 30 to 50 parts by weight is necessary in order to obtain a polyimide tube having a high thermal conductivity in the range of 0.50 to 0.55 W/m·K. However, as specifically described in Examples of Patent Document 4, when the mixing ratio of carbon nanotube is increased, a mechanical strength, such as the tear strength, of the polyimide tube significantly decreases.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 62-3980    Patent Document 2: Japanese Unexamined Patent Application Publication No. 3-25478 (corresponding to the specification of U.S. Pat. No. 5,182,606)    Patent Document 3: Japanese Unexamined Patent Application Publication No. 8-80580    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-123867